The knee joins the femur to the tibia and is controlled by ligaments and cartilage. Contact between the femur and the tibia occurs across the cartilage and there are so called compartments or spaces at each side of the knee. The medial compartment is on the inside of the knee, the lateral compartment is on the outside of the knee.
Unicompartmental osteoarthritis, which may occur in the medial compartment or in the lateral compartment, is a mechanical malfunction of the knee whereby uneven distribution of pressure occurs across the knee causing excessive wear on the inside of the knee joint in medial compartment osteoarthritis and on the outside of the knee joint in lateral osteoarthritis.
Unicompartmental medial osteoarthritis is most common in men. The male pelvis is somewhat narrower than the female pelvis and a male's legs tend to be varus, that is males have a greater tendency to bow-leggedness. Unicompartmental lateral osteoarthritis is most prevalent in females due to the wider pelvis of the female and the fact that the legs are more often in valgus (that is females tend to be knocked-kneed).
A healthy knee joint has an even distribution of pressure medially and laterally and the space between the femur and tibia is approximately one quarter inch.
Unicompartmental osteoarthritis can be induced by injury or by aging. With the advancement of the disease, the space between the femur and tibia decreases. The problem may progress to the extent that the space is eliminated and the femur contacts the tibia. In those circumstances, erosion of the tibia may result.
With the disease there is a change in the normal angle between the femur and tibia. For example, if the patient normally stands in 2.degree. of varus, that is bowleggedness, then with the advancement of the disease, the angle will increase to, for example, about 5.degree. of varus.
Lateral thrust upon heel strike often accompanies the increase in the mis-alignment of the femur and the tibia. This tends to stretch the ligaments on the opposite side as well as having an adverse effect on the knee joint, tending to emphasize the erosion of the tibia.
A further complication is that rotational slackness develops as the space between the compartment is reduced. This is caused by slackening of the ligaments as the attachment points of the ligaments move closer together with bone deterioration.